We have developed new and efficient routes to modify phosphines with phosphonic acid groups. Phosphonate-phosphines showed high solubilities in water and were used to immobilise rhodium catalysts in the aqueous phase of biphasic systems. In the two-phase hydroformylation of propylene, some of the novel catalysts showed activities and regioselectivities similar to those of Rh/TPPTS. Amphiphilic Rh/phosphonate-phosphine catalysts were found widely superior to Rh/TPPTS in the hydroformylation of 1-octene.